How To Choose Correct Backspacing With Forgeline

Let’s face it – everyone digs a good looking set of aftermarket wheels. Even Grandpa in his golf cart would slap on a set of polished billet wheels from a company such as Forgeline Motorsports if Grandma wasn’t keeping a menacing death grip on the social security check. Because when it comes to giving your vehicle that great look and stance that you always envisioned, sprucing up the appearance of a worn-out sled that’s a little worse for wear, or simply giving a ride that isn’t all that cool some semblance of cool factor, nothing goes further than a sharp new set of wheels to roll around town on.

However, tricking out your ride with a new set of wheels isn’t just a matter of picking out the set that tickles your fancy, swiping your credit card and bolting those bad boys on. There’s a reason that wheel manufacturers offer a plethora of wheel widths, diameters, offsets, and backspacing options. One size truly doesn’t fit all. The general trend in wheels these days, and certainly amongst those of us in the performance automotive market, is to stuff the largest, widest set of wheels and tires as we possibly can between the wheel wells. But before doing so, there are some proper steps that one can take to ensure they choose the proper wheel for their vehicle the first time, because as we all know, they aren’t cheap and once you mount the tires, you own them.

A wheel center is formed in a CNC machine at Forgeline.

powerTV spoke with Forgeline’s David Schardt to learn some of the insider tips and tricks to determining proper wheel backspacing, as well as making the correct wheel selection for your vehicle. Forgeline, based in Dayton, OH, has been producing some of the highest quality custom forged aluminum wheels on the market for more than 15 years.

Not only are these guys passionate about cars, but they are racers at heart as well. And that advantage provides Forgeline with insight into building wheels that meet a full spectrum of demands; from endurance road racing to your daily driver. The main characteristic of Forgeline wheels is their design for lasting strength and the utmost in performance, and because almost every wheel is custom designed, they’re experts at sizing as well. Oh, and they look pretty darn good, too.

What is Backspacing and Wheel Offset?

This side view diagram of a wheel provides a nice, simple look at the difference between backspacing and offset, which contrary to popular belief are not the same thing.

Before we move on, because backspacing is a commonly misunderstood attribute, let’s go over what it is. Backspacing is the measured distance from the inner bolting surface of the wheel to the outer edge of the inner side of the wheel. This is often and easily confused with wheel offset, which is defined as the distance from the exact center of the wheel to the inner and out edges of the wheel. These measurements are typically very closely related to one another.

When the backspacing – the mounting surface – is at wheel center, you have what is known as zero offset. When the mounting surface is behind (inboard) of the wheel center you have negative offset, and vice versa, when the mounting surface is outboard, you have a positive offset. What do these terminologies mean to you? Well, these dynamics and measurements are an integral part of wheel fitment, because not only do you have the size of the wheel wells and turning radius of the front wheels to contend with in making a sizing selection, but you also have components such as tie rod ends, A-arms, brake calipers, and shock and strut mounts to account for, as well. Let’s not forget that you also have the width of the tire as part of this equation.

This diagram shows nearly every characteristic of measurement of a wheel, including the front and rear spacing, negative and positive offset, ane the offset from the centerline.

Measuring Your Vehicle’s Available Space And Maximizing What You Have

The first step, or at least one of the first steps to determining the correct wheel width, is measuring to find out how much real estate you’ve got to work with in the front and rear fender wells. This calculation isn’t a determination of how wide of a wheel and tire you can use, it just allows you to know what’s available to you. If you measure it out to 10-inches, you probably aren’t getting an 11-inch wide wheel and tire combination in there. One of the simplest and easiest ways to measure this is by taping a string to the outer edge of the fender so that it hangs down a perfect 90 degrees to the floor, thanks to the laws of gravity. You can then use a tape measure or yard stick to measure in to the innermost point of contact, most commonly the shock or shock tower.

“Basically, what you’ll have to do is crawl under the car and determine this point,” says Schardt. “The shock and shock tower are commonly the closest point on the backside, but that isn’t necessarily always the case. You’ve got to determine what is going to come in contact with the back of that wheel.” Once you’ve determined that point and measured from there to the string you’ve placed on the fender, you’ll then have your total width. From there, you can measure from the string to where the wheel sits to give you the front space. You can then do the simple math of subtracting that distance from the total width to arrive at your backspacing measurement. “Backspacing is backspacing – people really make it more complicated than it is,” continued Schardt. “It’s simply how much rim you can get from the back of the mounting pad to whatever is going to hit the wheel first on the back. That’s all it is.”

The custom Forgeline S03P 19-inch wheels on our Pontiac G8 GXP.

Sounds simple enough, but there are a few suggestions to keep in mind prior to this step.

Now obviously, the only logical way to conduct such measurements is to remove a wheel and jack the car up in the air or place it on a lift. However, that leads to one of the most common mistakes of improper measurement and subsequently, the wrong wheels. Because now you have the suspension hanging in a manner unlike that of when the weight of the car is upon it. “When the suspension is hanging, the geometry is completely wrong,” explained Schardt. “After you jack up the car, maybe put a jackstand or something under the brake rotor and let the car back down to where its close to ride height, and then take your measurements from there. Measuring the car with the suspension hanging changes your outcome considerably.”

Some forward thinking in all of this is a great idea, as well. The weight of passengers in your vehicle or equipment in your trunk will alter the ride height and depending on your vehicle, the width of space available in the fender well and between the suspension components. With the car jacked up and the the brake rotor supported as David suggested, asking a couple of your drinking buddies to pile in the car while you perform your measurements could certainly help avoid the tires coming in contact with things you don’t want them to contact later.

Something you always need to account for when selecting a set of wheels if allowing enough space for the brakes. Forgeline's David Schardt suggests planning out your brake package before choosing a wheel, rather than vice versa.

David went on to explain that often times, the best way to measure for backspacing is by using an existing wheel as a reference point, “It’s easier for many people if they have an existing wheel to put up on there that happens to fit, even if they have to put it on with spacers. They can then determine what the backspacing of that wheel is and then measure off of that. Say for example, you have an 8-inch wheel and it’s got 4-inch backspacing, you can measure from the edge of the rim to the inside shock point or closest point and that’s 3-inches. Even if it’s just an 8-inch wheel that you’re measuring but you want a 10-inch wheel, you can still use it as a good reference point. This method is a lot easier for many people instead of just measuring air.”

As with anything, getting all your ducks in a row from the outset will help alleviate a lot of headaches down the road if you’re in the midst of building a project car. It’s much easier to fit your wheels around the necessary components under the car than it is to purchase the wheels first and then force yourself to fit everything else to them. If it’s even possible at all.

To measure the backspace of a wheel, simply place the wheel face down on the ground and with a straight edge laid across the lip of wheel, measure down to the center of the wheel where it bolts to the hub.

“We get a lot of customers that don’t have all of that together, but if they know what brake package they are going to run and how much distance they have in there to work with, we can typically get measurements from the brake companies,” says Schardt. “We have diagrams for several of the top brake companies and if not, we can get them.”

Consideration for Brake Upgrades

The above diagram displays the characteristics of and measurements necessary for allowing the optimum clearance for large brake and caliper kits. Point "A" is the distance from the face of the axle pad inward, while "B" shows the distance from the wheel centerline to the inner edge of the caliper and C" from the wheel centerline to the outer edge of the caliper.

One important thing to note when picking our both your wheels and brakes is to do your homework to make certain they are compatible. While the brake kit won’t have an effect on your backspacing, it will play a role in which and what size wheel you can bolt up. Said Schardt, “It’s very aggravating to us, but the brake companies don’t care about the wheel companies. People want bigger and bigger brakes and so they manufacture these huge brakes that stick out, and then people can’t find wheels for them. So it’s most advisable to decide on your wheel before you decide on the brakes.You can buy a large brakes but you may cut down your choices of wheels in doing so.”

If at a later time moving the wheels outward is desired or necessary, be it for looks or to clear new components, the option of using spacers to accomplish this does exist. Spacers are acceptable to a certain degree. You want to try to keep the wheel on the center hub in most cases; it isn’t critical but it’ll reduce the chance of vibrations, especially on some of the newer-style suspension cars.

Forgeline wheels make any car look great, and by taking the proper steps in selecting the optimum sizing, you can achieve just the right stance and maximize the amount of wheel you can fit under the fenders.

Borrowed from the Wheel Fitment section on the Forgeline website, this side view diagram pinpoints the wheel measurements that need to be take into account for wheel fitment when submitting their supplied sizing form.

Do The Math…Before You Buy

While many of these tips may go a long way to helping you determine wheel and backspacing selection on your own, the folks at Forgeline won’t leave you to fend for yourself in making that determination. On the Forgeline website, you’ll find a section titled “Wheel Fitment” that provides a diagram of the hub and brake assembly which you can use as a guide for measuring your own vehicle, and an accompanying form to complete with information regarding the make and model and other attributes of your vehicle.

Factors that you’ll need to measure include the front and rear spacing, caliper clearance, caliper height, hub diameter, hub height, and other vehicle information. Technicians at Forgeline can then use those elements to determine the correct wheel for your needs. Thus, whether you’re completely clueless or just looking for a second opinion from the experts, you can rest easy knowing you aren’t left to fend for yourself in your measurements and selection.

Hopefully the pointers and suggestions in this article will ease your concerns of measuring for and selecting wheels by simplifying what really is a simple process. As always, proper planning in regards to your components and taking all the necessary precautions to avoid improper measurements will go a long way to getting things right the first time. Because if you’re like us, you’re probably impatient about getting those sharp new wheels on the road, rather than dealing with the hassle of returns or modifications.

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About the author

Andrew Wolf

Andrew has been involved in motorsports from a very young age. Over the years, he has photographed several major auto racing events, sports, news journalism, portraiture, and everything in between. After working with the Power Automedia staff for some time on a freelance basis, Andrew joined the team in 2010.